1. Field of the Invention
The present invention relates to an apparatus and a system both for driving fuel injectors that have piezoelectric elements, and, in particular, to the apparatus and the system that are preferably adapted to vehicles with diesel engines serving as drive sources.
2. Description of the Related Art
An apparatus for driving injectors of a diesel engine, i.e., an injector driving apparatus, is provided with a power source composed of such members as a DC-DC converter and a charge unit charging piezoelectric elements based on the power from the power source. The power source receives voltage (12V) from a battery to boost it up to a higher voltage of several dozens to several hundreds volts. The charge unit receives the boosted higher voltage to charge the piezoelectric elements by causing current to repeatedly flow from the power source to the piezoelectric elements for a specific period of time. This way of charging the piezoelectric elements is exemplified by Japanese Patent Laid-open Publication No. 2002-136156.
However, the injectors are manufactured to perform fuel injection every time an injector is charged with energy of a given level or more. The energy E charged by a piezoelectric element can be expressed by a formula of “E=(½)(CV2),” where C is a capacitor of the piezoelectric element and V is a voltage to be applied to the piezoelectric element.
In recent years, to reduce harmful substances contained in exhaust gas, it is absolutely necessary to precisely control the intervals of fuel injection. This way of fuel injection control increases the number of times of operating the injectors (i.e., fuel injection number).
In this apparatus, the voltage of the power source will decrease due to charging the piezoelectric elements, and will not be restored to its original value until a predetermined period of time passes.
Hence, when an interval of time between adjacent fuel injection operations of each injector becomes smaller so as to provide “successive (or nearly contiguous)” injecting operations, the voltage of the power source may not be restored to its original value. If such a case happens, as apparent from the foregoing formula, the amount of electric energy accumulated in each piezoelectric element is forcibly smaller than a voltage which is a control target value for the injector driving apparatus.
In the following description, the “successive fuel injection” means that each injector injects the fuel successively a plurality of times during one air-intake stroke carried out by each cylinder of the internal combustion engine. When this “successive fuel injection” causes a shortage in the charged energy compared to its target value, this is called “influence of the successive fuel injection.”
Further, the capacitance of the piezoelectric element fluctuates depending on the temperature. Such fluctuations will also cause a problem that, as apparent from the foregoing formula, the amount of electric energy accumulated in the piezoelectric element is deviated from its target value.
In addition, it is inevitable that products of the injectors and injector driving apparatus have fluctuations of performances (i.e., individual differences). This problem may also result in a deviation, from its target value which needs to be controlled, of the amount of electric energy accumulated in each piezoelectric element by the injector driving apparatus.
The above situations may cause a problem in that the amount of fuel injected by each injector also deviates from a desired amount to be injected.